The invention is concerned with aqueous dispersions of fluoropolymers being essentially free of fluorine-containing emulsifiers, a process for preparing such dispersions and their use. xe2x80x9cEssentially freexe2x80x9d means a content of less than 100 ppm, preferably less than 50 ppm, especially less than 25 ppm and in particular less than 5 ppm.
Polyfluoroethylene dispersions find wide application in the coating industry due to the unique performance of the coatings in respect of e.g. release properties, good weathering resistance, and flame retardancy. They are mainly used for coating kitchenware, chemical apparatus and glass fabrics. In many such applications, the dispersions are applied at relatively high solids contents, e.g., up to 70% by weight. These concentrated dispersions are mainly stabilized by nonionic emulsifiers such as alkylarylpolyethoxy alcohols and alkylpolyethoxy alcohols, using colloid-chemistry methods.
There are in principle two different polymerization processes for preparing fluoropolymers, namely suspension polymerization leading to polymer granules and, on the other hand the process known as emulsion polymerization, leading to an aqueous colloidal dispersion. This invention concerns emulsion polymerization, the resultant dispersions and their use.
The manufacturing of such dispersions involves in principle the two processing steps polymerization and concentration.
Polymers which are obtainable by aqueous emulsion polymerization are firstly homopolymers not processible from the melt, for example PTFE, secondly xe2x80x9cmodifiedxe2x80x9d polymers, for example a polymer with more than about 99 mol % of tetrafluoroethylene (TFE) and an amount of comonomer(s) which is so low that the product retains its xe2x80x9cnot processible from the meltxe2x80x9d character and thirdly low-molecular weight xe2x80x9cmicropowderxe2x80x9d dispersions which are processible from the melt, and fourthly copolymers, for example fluorinated thermoplastics or fluoroelastomers. The fluorinated thermoplastics include copolymers which are composed mainly of TFE and the amount needed of one or more comonomers to make the product processible from the melt, for example from 1 to 50 mol %, preferably from 1 to 10 mol %. Customary fluoromonomers, besides TFE, are vinylidene fluoride (VDF) other fluorinated olefins, such as chlorotrifluoroethylene (CTFE), in particular perfluorinated olefins having from 2 to 8 carbon atoms, such as hexafluoropropene (HFP), fluorinated ethers, in particular perfluorinated vinyl alkyl ethers whose alkyl moieties have from 1 to 6 carbon atoms, for example perfluoro (n-propyl vinyl) ether (PPVE). Other comonomers which may be used arc nonfluorinated olefins, such as ethylene or propylene. The resultant dispersions of polymers which are processible from the melt or not processible from the melt generally have a solids content of from 15 to 30% by weight. To achieve the abovementioned high solids content for application as a coating, and advantageously also for storage and transport, the solids content has to be increased by raising the concentration. Examples of methods used for this are raising the concentration thermally as in U.S. Pat. No. 3,316,201, decanting (U.S. Pat No. 3,037,953) and ultrafiltration (U.S. Pat. No. 4,369,266).
The known emulsion polymerization mostly takes place within a pressure range from 5 to 30 bar and within a temperature range from 5 to 100xc2x0 C. as described in EP-B-30 663, for example. The polymerization process for preparing PTFE dispersions substantially corresponds to the known process for preparing fine resin powders, known as paste product (U.S. Pat. No. 3,142,665). The polymerization process for preparing copolymers, such as dispersions of fluorinated thermoplastics, corresponds to the process for preparing these materials in the form of melt pellets.
In all of these emulsion polymerizations an emulsifier is required which does not disrupt the polymerization by chain transfer. These emulsifiers are termed nontelogenic emulsifiers (U.S. Pat. No. 2,559,752). Use is mainly made of perfluorooctanoic acid (PFOA for example n-PFOA, CAS No. 335-67-1) in the form of ammonium and/or alkali metal salts. However, the abbreviation PFOA when used in the text below is not intended to exclude other fluorinated emulsifiers. The content of this emulsifier is generally within the range from 0.02 to 1% by weight, based on the polymer.
Occasionally, other fluorinated emulsifiers are used. For example, EP-A-822 175 describes the use of salts of CH2-containing fluorocarboxylic acids for the emulsion polymerization of TFE. WO-A-97/08214 describes the use of 2-perfluorohexylethanesulfonic acid or salts thereof for TFE polymerization.
U.S. Pat. No. 2,559,752 describes other fluorinated emulsifiers, but these have not been widely used since their volatility is low. These chemicals can cause discoloration of the final products at high processing temperatures.
One of the greatest advantages of PFOA is its high volatility. PFOA is a very effective emulsifier and is practically indispensible due to its inertness in the polymerization reaction. However, PFOA is not biodegradable and has recently been classified as hazardous to the environment.
However, it is known that PFOA can be removed from exhaust gases (EP 731 081), and moreover advantageous processes for removing PFOA from wastewater have been described (U.S. Pat. No. 4,282,162 and the as yet unpublished German Patent Applications 198 24 614.5 and 198 24 615.3 filed on Jun. 2, 1998).
In the techniques listed above for raising concentration, the majority of the PFOA remains in the polymer dispersion, even in the case of ultrafiltration or removal by decanting using a 100-fold excess of the nonionic emulsifier.
For example, in the ultrafiltration of U.S. Pat. No. 4,369,266 about 30% of the initial PFOA content remains in the marketable dispersions. In specific cases the residual PFOA content can be reduced to below 10%, but the process is generally not cost-effective: achieving a reduction of this type requires addition of water and of a nonionic emulsifier to the dispersion whose concentration is to be raised. This gives unacceptably long process times.
During subsequent use of these dispersions, PFOA can pass into the environment, for example with the wastewater inevitably arising from cleaning the equipment, and into the atmosphere as aerosol. The latter emission is still more pronounced when coatings are produced, since PFOA and its ammonium salt are highly volatile. In addition, PFOA and its salts decompose by decarboxylation at the sintering temperatures normally employed, from 350 to 450xc2x0 C., to give fluorinated hydrocarbons, which have a major global-warming effect (xe2x80x9cgreenhouse effectxe2x80x9d).
The present invention provides high solid dispersions essentially free of PFOA. In this invention, xe2x80x9cessentially freexe2x80x9d means a content of less than 100 ppm, preferably less than 50 ppm, especially less than 25 ppm and in particular less than 5 ppm. These values are based on the entire dispersion, and not just the solids content. This is achieved by removal of fluorinated emulsifiers, e.g. PFOA, from fluoropolymer dispersions, such as PTFE, fluorothermoplast or fluoroelastomer dispersions, via anion exchange, namely by adding a nonionic emulsifier to the fluoropolymer dispersion and contacting this stabilized dispersion with a basic anion exchanger. This process works without jamming or clogging the ion exchange bed by coagulated latex particles. The resulting dispersion may optionally be upconcentrated.
Fluoropolymer dispersions useful in this inventions include dispersions of homopolymers and copolymers of one or more fluorinated monomers, such as TFE, VDF or CTFE or other fluorinated olefins of 2 to 8 carbon atoms, perfluorinated olefins of 2 to 8 carbon atoms, e.g. HFP, fluorinated ethers, especially perfluorinated vinyl-alkyl ethers with alkyls of 1 to 6 carbon atoms, such as perfluoro-(n-propyl-vinyl) ether and perfluoro-(methyl-vinyl) ether. Useful comonomers also include non-fluorinated olefins, such as ethylene and propylene. The invention is intended to include such dispersions whether the resulting fluoropolymer is melt-processible or not.
The latex particles usually have a submicroscopic diameter of less than 400 nm and preferably from 40 to 400 nm. Smaller particle sizes may be obtained by what is known as xe2x80x9cmicro-emulsion polymerization.xe2x80x9d The latex particles are anionically stabilized by colloid chemistry methods. The anionic stabilization is provided by anionic end groups, mostly COOH groups, and by the anionic emulsifier, such as PFOA. Such anionically stabilized dispersions coagulate rapidly in an anion exchange bed and thus jam the ion exchange bed. The reason for that is the breakdown of the electrical double layer at the ion exchange sites. The treatment of an anionically stabilized dispersion with an anion exchanger is therefore considered to be technically not feasible, in particular for higher concentrations.
The impairing or clogging of the ion exchange bed is observed even at concentrations 1000 times lower than those of the raw polymer dispersions, that is to say of the dispersion after polymerization.
A helpful observation in choosing a useful ion exchanger is that the pKa value of the acid corresponding to the counterion of the anion exchanger has to be higher than the pKa value of the anionic end groups of the polymer. Preferably, the anion exchanger has a counterion corresponding to an acid with a pKa value of at least 3.
In contrast, coagulation is observed after prolonged periods if the anion exchanger is in the SO4xe2x88x922 or Clxe2x88x92 form, even with dispersions of copolymers of TFE and HFP, called xe2x80x9cFEPxe2x80x9d, and of TFE and PPVE, called xe2x80x9cPFAxe2x80x9d. These copolymers both have strongly acidic end groups. The formation of such end groups is explained in xe2x80x9cModern Fluoropolymersxe2x80x9d, John Scheirs (Editor). John Wiley and Sons, Chichester (1997), pages 227 to 288, 244. The jamming or clogging of ion exchange beds when processing TFE-ethylene or VDF copolymer dispersions occurs almost instantly under such conditions.
Therefore, at the outset, the anion exchange is performed in an essentially basic environment. Preferably, the ion exchange resin is transformed to the OHxe2x88x92 form, but anions like fluoride or oxalate corresponding to weak acids can also be used. These anions are generally present in the dispersion and originate from the polymerization recipe.
The specific basicity of the anion exchanger used is not critical. Strongly basic resins are preferred due to the observed higher efficiency in removing PFOA. The effective removal of PFOA from the dispersions depends on the ion exchange conditions. Weakly basic ion exchange resins show earlier PFOA breakthrough. The same is true for higher flow rates.
The flow rate is not critical, standard flow rates can be used. The flow can be upward or downward.
The ion exchange process can also be carried out as a batch process by mildly stirring the dispersion with the ion exchange resin in a vessel. After this treatment the dispersion is isolated by filtration. Use of this invention will minimize coagulation during a batch process.
Non ionic emulsifiers are described in detail in xe2x80x9cNonionic Surfactantsxe2x80x9d M. J. Schick (editor), Marcel Dekker, Inc., New York 1967.
The choice of the nonionic emulsifier is also not critical. Alkylarylpolyethoxy alcohols, alkylpolyethoxy alcohols, or any other nonionic emulsifier can be used. This is a big advantage since the removal of PFOA from commercial dispersions leaves the formulation of the applied dispersions essentially unchanged.
No differences could be observed using nonionic surfactants such as those of alkylarylpolyethoxy alcohol type, e.g., Triton(trademark) X100, or of alkylpolyethoxy alcohol type, e.g., GENAPOL(trademark) X 080, with respect to effectiveness of the PFOA removal, flow rates, or jamming of the ion exchange bed.
The removal of PFOA is preferably carried out with crude dispersions from polymerization. Such dispersions generally have a solids content of 15 to 30% by weight. Sufficient nonionic emulsifier is added to provide dispersion stability during subsequent processing, such as concentration. A sufficient quantity of nonionic emulsifier generally means from 0.5 to 15% by weight and preferably from 1 to 5% by weight. These percentages are based upon the solids content of the dispersion. After removal of the PFOA, the dispersions may be concentrated using conventional procedures, such as ultrafiltration or thermal concentration. It is advantageous that the concentration of the nonionic emulsifier in the final product is not much higher than in comparable commercial products. The absence of PFOA in these processes does not impair the concentration process, that is, no more coagulum is formed than in the presence of PFOA during thermal concentration and ultrafiltration.
The removal of PFOA via anion exchange can also be carried out with previously concentrated dispersions with a solids content of up to 70% by weight. However, due to the higher viscosity and density of such dispersions this process is technically more cumbersome. In this case the ion exchange is preferably operated by the upflow method, to avoid difficulties due to the flotation of the ion exchange bed. The high viscosity does not usually permit high flow rates. For such high solids dispersions the batch process appears to be more advantageous.
The removal of PFOA is carried out by adding typically 1-5% by weight of nonionic emulsifier to the dispersion under mild agitation conditions and passing the dispersion over the anion exchanger. The anion exchanger may be preconditioned with a solution of nonionic emulsifier as used with the dispersion to be exchanged. The anion exchange resin is preferably brought into the OHxe2x88x92 form. This is accomplished by bringing the anion exchange resin into contact with an NaOH solution.
Dispersions are generally used for the ion exchange process without adjusting the pH value but the pH value may also be increased to enhance the colloidal stability of the dispersion by adding a base, such as aqueous ammonia or sodium hydroxide solution. A pH value in the range of 7 to 9 is sufficient. The increased pH value does not greatly affect the efficiency of the removal of PFOA. This is believed to be due to the fact that PFOA is not only exchanged but also strongly absorbed on the ion exchange resin.
Subsequently the ion exchanged dispersions are subjected to concentration, preferably using thermal concentration or ultrafiltration. No impairment of these processes could be observed. There are moreover no changes in end user processing or end use properties for such dispersions according to the invention.
The anion exchange process in the presence of a nonionic emulsifier, without jamming of the ion exchange bed, can be successfully used for the removal of any other anionic emulsifier used in any polymerization process.
This process may also be used for any crude fluoropolymer dispersions, such as a dispersions of PFA, FEP, THV (THV is a terpolymer of TFE, HFP and VDF), ET (ET is a copolymer of TFE and ethylene), TFE/P (a copolymer of TFE and propylene), copolymers of VDF and HFP, as well as homopolymcrs or copolymers comprising other fluorinated olefins or vinyl ethers. These polymers are described in detail in xe2x80x9cModern Fluoropolymersxe2x80x9d, cited above.
The work-up procedure as disclosed in U.S. Pat. No. 5,463,021 describes inter alia a treatment of crude THV dispersions via an ion exchange process as one work-up step. However, this is a cationic exchange process to remove manganese ions originating from the permanganate used as polymerization initiator. During the cationic exchange process the stabilizing electrical double layer is not affected because the latex particles are anionically stabilized.